1. Field of the Invention
The present invention generally relates to coaxial connectors and, more particularly, to a coaxial connector used for connection between circuit boards in a high-speed optical module or a radio module, and a connector assembly, a printed circuit board and an electronic apparatus having such a coaxial connector.
2. Description of the Related Art
For example, in an optical transceiver module, which processes a several 10 GHz high-speed signal, a coaxial cable is used as a transmission line, which connects between circuit boards or between devices. The coaxial cable has advantages in that it has a small dielectric loss with respect to a signal, it has a certain degree of flexibility and it can be easily handled. Accordingly, when connecting one circuit board to another circuit board by a transmission line, even if connecting portions of the circuit boards are not arranged along a straight line, they can be connected by a coaxial cable being bent in a necessary form.
The coaxial cable has a three-dimensional structure in which an inner conductor is surrounded by an outer conductor. On the other hand, a transmission path on a circuit board has a planer structure in which outer conductors are arranged on both sides of an inner conductor, or a so-called micro-strip structure in which an inner conductor and an outer conductor are arranged on front and back surfaces of an insulating film, respectively. Thus, a coaxial connecter for connecting a coaxial cable and a transmission line on a circuit board is configured to convert the three-dimensional structure of the coaxial cable into the planer structure of the circuit board.
As a coaxial connector attached to end portion of a circuit board, there are a screw-shaped connector (for example, SMA, V, etc.) and a push-on connector (for example, SMP, GPPO, etc.). Additionally, there is suggested a connector structure for converting into a micro-strip line path, in which a signal line formed on a surface of a board faces an inner conductor of a coaxial line path and a grounding line formed on a back surface of the board faces an outer conductor (for example, refer to Patent Document 1).
Patent Document 1: Japanese Laid-Open Patent Application No. 5-235613
In recent years, miniaturization of optical transceiver modules and microwave transmission apparatuses has progressed, and there is a demand for high-density mounting in those apparatuses. Although a coaxial cable has a certain degree of flexibility, an allowable bending radius of the coaxial cable is about several 10 mm, and it cannot be bent at a smaller bending radius. For example, if a coaxial cable is bent in a bending radius of several 10 mm or less, there is a possibility that a distortion occurs in an outer conductor, which may degrade a frequency characteristic of the coaxial cable. In a worst case, the outer conductor may be folded down and contacts the inner conductor.
Therefore, even if an attempt is made to increase packaging density by arranging the circuit boards and devices close to each other, it is necessary to provide a certain distance between circuit boards and between devices so as to acquire an allowable bending radius of a coaxial cable. Moreover, it is necessary to arrange the circuit boards and devices so that a bending radius of a line along which a coaxial cable is extended becomes large, which results in the coaxial cable being disincentive when an attempt is made to improve the packaging density of circuit boards and devices.
It is considered that the connectors of the circuit boards or the devices are connected directly without using a coaxial cable. However, in such a case, if one of the circuit boards or devices is fixed, the connector of the other circuit board or device must be moved in a radial direction, which requires a structure in which the connector is movable. Additionally, if both circuit boards or devices are directly connected by connectors and both circuit boards or devices are fixed, and when a stress is generated in the connecting portions (connectors) due to a temperature fluctuation, such a stress cannot be relaxed and there may be a fluctuation in the connecting portion characteristic due to the temperature fluctuation.